Edit: I'm giving up on my attempts to hack SFX / Music together. My general idea was to drop one channel from the music, which'd free up a channel for sound (it'd obviously mean that you'd lose an entire instrument but I figured the SFX would be a worthwhile substitute).

However there's a loop that sets all of the channels when a song is playing, my attempts to cleanly reduce the length of that loop from 4 to 3 iterations haven't been successful. Maybe someone smarter than me can work it out!